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Section 5
Propagation
5C1 Recall that hills cause radio shadows and that signals become weaker as they penetrate buildings
Radio waves are similar to light. At VHF and
UHF, hills cause "shadows" with only a weak signal reaching the far
side of the hill if at all!. Radio waves also are weaker after
penetrating buildings, but glass windows are more transparent to
radio waves. Solid materials (e.g. walls) absorb and adversely
weaken the radio wave.
The range achieved at VHF/UHF is dependent on antenna height, a
clear path and transmitter power. Higher antennas are preferable to
higher power, because a higher antenna improves both transmit and
receive performance. Outdoor antennas perform better than indoor
antennas. The surrounding walls reduce the effectiveness of an
indoor antenna.
Recall that at VHF/UHF, range decreases as frequency increases and
that in general VHF/UHF waves have a range not much beyond line of
sight.
At VHF/UHF, range decreases as frequency
increases. In general, VHF/UHF waves have a range not much beyond
"line of sight". Line of sight is how far you can see. The range of
VHF/UHF may be slightly beyond line of sight due to refraction at
the horizon. Refraction refers to the bending of the radio wave.
VHF and UHF are normally used for local communications, whereas HF
is suitable for long distance communication.
Higher frequencies, that is VHF and UHF, are not reflected by the
ionosphere but pass straight through it. VHF and UHF signals go
straight through the ionosphere and are lost in space [unless
communicating with a satellite or spacecraft]. Therefore, VHF and
UHF radio waves are normally used over short distances i.e. line of
sight.
Recall that sporadic E and atmospheric ducting can increase the range
of VHF and UHF signals.
In the troposphere there are also layers of gases and the E layer when ionised can act as a reflector for VHF and UHF and extend the distance at which the signals can be received and similarly with a phenomenon called DUCTING where the signal passes through layers which have trapped to signal much like water flowing through a pipe.
5C2 Recall that falling snow, hailstones and heavy rain can attenuate signals at UHF and above.
Amazing as it might seem but falling snow, hailstones and heavy rain can attenuate signals at UHF and above.
5C3 Recall that the range achieved at VHF/UHF is dependent on antenna height, antenna gain, a clear path and transmitter power
Raising the antenna above obstructions is
likely give a clear path for the radio waves to travel. On VHF/UHF,
if loss is high, a clear path is much more effective in getting a
good signal than a 10 or 100 times increase in transmit power. For
example, satellites can be accessed with very low power at a great
distance. Typical ranges are a few km with a handheld, and tens of
km from a mast-mounted antenna.
Obstructions (e.g. hills and buildings) between two radio stations
can block the radio signal, making communication impossible on
VHF/UHF. In contrast, where there is a clear path between two radio
stations, for example between 2 ships at sea, communication is
possible on VHF/UHF.
Understand that higher antennas are preferable to higher power as they improve both transmit and receive performance.
Higher antennas are preferable to higher power, because a higher antenna improves both transmit and receive performance.
Recall that outdoor antennas will perform better than indoor antennas
Outdoor antennas perform better than indoor antennas as the signal is radiating out into the air and does not have to pass though walls, windows or doors or the roof to start propagating.
The origin of some of the text on this page is from the RSGB with additions by the web master
